I'm speaking to pre-2nd ed D&D ie OD&D, Moldvay Basic, AD&D (and presumably Holmes Basic as well, but I don't know it except by reptuation).Are you speaking about AD&D or OD&D here? Because, AD&D doesn't agree with you. Heck, the Appendix in the AD&D PHB outlines the outer planes and tells you that there are lots and lots of gods.
Now, is it easy to do? Maybe. I don't know. But, the point is, D&D has NEVER been presented that way.
And what I'm saying is that - regardless of what Appendix IV of the PHB says - it is trivially easy to approach the game in broadly monotheistic terms, because the clerics and paladins are straight from some pulpy idealisation of the Knights Templar and their ilk.
Whereas, as soon as you try to actually implement Appendix IV, you get the problem that clerics of the gods of the Happy Hunting Grounds - who should, presumably, be shamanistic types like the 4e primal Shaman - look like they all stepped out of the pages of some mediaeval romance, with their heavy armour and heavy weapons and healing touch and the like.
This was a very frequent issue of discussion in Dragon magazine. Gygax tackled it, to some extent, by giving GH clerics special abilities and restrictions that reflected their gods (and he has a Dragon editorial in no. 92 about this). But it was 2nd ed that actually tried to provide a systematic solution, through the idea of specialty priests with sphere-based spell access.
Since 2nd ed, the only version of D&D that has really reverted to the earlier way of presenting clerics and paladins has been 4e (which even went so far as to make druids, shamans etc not servants of the divinity at all.
I'm not asserting that D&D is (or was) monotheistic. I'm asserting that the pre-2nd ed presentation of the divine classes makes it easy to play a game where all the clerics and paladins are templar/round table types (except for the evil high priests, anti-clerics and the like, who are members of dark cults that serve demons and other evil spirits), and will actively get in the way if you want to actually implement Appendix IV (with priests of Poseidon and Zeus and Odin and Baldur and Freya and etc).the idea that D&D is monotheistic because clerics and paladins
And Moldvay Basic doesn't even have Appendix IV. As I said, one of the first PCs I GMed was a cleric, and I don't remember any details of how we handled religion, but as best I recall the general details, he was just a servant of the divinity. He certainly didn't look like a priest serving a member of one of the classic pantheons.